As is known, in integrated or “embedded” PCM memories, the storage of the information takes advantage of the characteristics of materials which have the property of switching between phases having different electrical characteristics. Such materials can switch between a disordered amorphous phase and an ordered crystalline or polycrystalline phase; with each phase is associated a significantly different value of resistivity, and consequently a different value of the stored data.
For example, the elements of group VI of the periodic table, such as Tellurium (Te), Selenium (Se), or Antimony (Sb), called chalcogenides or chalcogenic materials, are advantageously usable for the formation of phase-change memory cells; in particular, an alloy composed of germanium (Ge), antimony (Sb) and tellurium (Te), known as GST (having the chemical composition Ge2Sb2Te5) may be used in such memory cells.
The changes of phase may be obtained by locally increasing the temperature of the cells of chalcogenic material, through resistive electrodes, generally known as heaters, disposed in contact with respective regions of chalcogenic material.
Access or selection elements, in particular MOS transistors, are connected to the heaters, and selectively enable the passage of a programming electrical current; by Joule effect, this electrical current generates the temperatures necessary for the change of phase. In particular, when the chalcogenic material is in the amorphous state, at high resistivity (the state referred to as RESET), it is necessary to apply a current/voltage pulse (or a suitable number of pulses) of duration and amplitude so as to allow the chalcogenic material to cool down slowly. Being subjected to this treatment, the chalcogenic material changes its state and switches from the state of high resistivity to a state of low resistivity (the state referred to as SET). On the other hand, when the chalcogenic material is in the SET state, it is necessary to apply a current/voltage pulse of suitable duration and high amplitude in such a manner as to make the chalcogenic material return to the amorphous state at high resistivity (RESET state).
In read mode, the state of the chalcogenic material is detected by applying a voltage that is sufficiently low so as not to cause it to substantially heat up, and thus reading the value of the current that flows in the memory cell (which is again selected by means of the respective access MOS transistor). Given that the current is proportional to the conductivity of the chalcogenic material, it is possible to determine the state the material is in, and thus to gain access to the data stored in the memory cell.
In general, PCM memories offer significant advantages, amongst which are a high scalability and a high read speed combined with a reduced current consumption and a high efficiency; these advantages mean that, at least in some sectors, it is acceptable to consider that PCM memories may replace non-volatile memories of the conventional type, for example of the Flash type.